ABSTRACT
Cardiac malposition describes an abnormal location of the heart. The concept of cardiac malposition includes abnormalities of overall cardiac position (dextrocardia, mesocardia, isolated levocardia in situs inversus), heterotaxy syndromes, pericardial defects, and ectopia cordis. Some knowledge of the embryology of heart formation facilitates understanding of the disorder underlying cardiac malposition. At a very early stage of development, when the embryo shows a perfect bilateral symmetry, the interaction of some genes determines an asymmetry in the structure and position of the thoracic and abdominal organs. The bulboventricular loop is usually considered to be the first visual evidence of asymmetry within the embryo with the development of the left (LV) and right (RV) morphologically different ventricles. Under normal conditions, the primitive heart tube loops to the right (dextro- or D-) forming a D-loop. The morphologic RV develops from the bulbus cordis, whereas the morphologic LV from the ventricle of the bulboventricular loop, so the direction of the initial formation of the cardiac loop determines the relative locations of the ventricles. Thus, in formation of a D-loop, the morphologic RV is to the right of the morphologic LV. The position of the atria is not influenced by the direction of the bulboventricular loop since the two atria at this stage are “anchored” regions of the heart and their final location depends upon their original location in the primitive tube [1]. The atrial situs is not affected by the shape of the bulboventricular loop or by the final locations of the ventricles. Thus, the atrial situs corresponds to the visceral situs, and abnormalities of atrial position are usually associated with abnormalities in the situs of other organs [1]. At the sixth embryonic week, the heart occupies a median position almost symmetrically perpendicular to the long axis of the body. After this early stage, the heart rotates leftward and this process is responsible for levocardia. Every anomaly in this process leads to the cardiac malposition which may range from a simple abnormal position of the heart to the loss of the lateral asymmetry of the thoracic and abdominal organs (isomerism). Since fetal dextrocardia may be associated with a wide spectrum of cardiac defects that are often complex, this condition is conceptually difficult to understand and diagnose. Therefore, a careful segmental analysis of cardiac anatomy is important to analyze and define the anatomic relationships.
